The present invention is related to low dielectric constant dielectric materials suitable for use, without limitation, in multilayer printed circuit boards.
The principal conventional dielectric material in present use for printed circuit boards is a laminated composite comprised of fiberglass fabric impregnated with a thermosetting epoxy resin, referred to by the National Electronic Manufacturers Association (NEMA) classification as FR-.sub.4. FR-.sub.4 is produced by impregnating fiberglass fabric with a liquid, thermosetting epoxy resin. The resin in the impregnated fabric is partially cured with heat to form a dry, flexible sheet in which the resin is in an intermediate cure state, termed "B"-stage or "pre-preg" sheet. One or more sheets of pre-preg are then stacked together to a desired thickness and laminated together by further curing under heat and pressure to form a laminated composite in which the resin is in a fully-cured state, termed the "C"-stage state.
During the lamination process, the B-stage epoxy resin of the pre-preg sheet is converted to fully-cured C-stage resin. Normally, the sheets of pre-preg are bonded to one or two sheets of copper foil during the lamination process so that the laminated composite consists of dielectric material clad on one or both sides with copper foil. This composite material is referred to as the FR-.sub.4 copper clad laminate and is fabricated into single and double sided printed circuit boards.
Where very high circuit densities are required, printed circuit boards with more than two layers of circuitry have been developed, called multilayer printed circuit boards. Thin dielectric FR-.sub.4 copper clad laminate is fabricated into single or double sided circuit patterns called innerlayers. One or more of these innerlayers are interleaved with one or more sheets of B-stage pre preg and laminated together under heat and pressure to form a homogeneous, void free multilayer structure. The lamination process converts the B-stage epoxy resin of the pre-preg into C-stage resin, bonding the innerlayers together and providing insulation between the circuit layers. The multilayered structure is further processed into a multilayer printed circuit board.
The thermosetting resin is essential to the fabrication of multilayer printed circuit boards in which the resin is uniform throughout. The B-stage resin in the pre-preg is converted to a fully-cured C-stage state without melting or materially altering the C-stage dielectric material in the innerlayers. Because the intermediately cured pre-preg is essentially the same composition as the C-stage resin in the innerlayers, the multilayer composite is dimensionally stable and easily processed.
There is a need in the electronics industry for dielectric materials having lower dielectric constants than that of convention materials because signal speeds and operating frequencies of electric systems have increased dramatically. Lower dielectric constants dielectric materials both decrease capacitive coupling and increase the speed of the electronic signal so that electronic systems can process data at greater speeds.
FR-.sub.4 laminate has a relatively high dielectric constant, approximately 5.0 at 1 megahertz. This is a result of the high dielectric constant contribution of the fiberglass, 6.11, averaged with the lower dielectric constant of the epoxy resin, 3.4. To achieve a lower dielectric constant dielectric material, the electronics industry has turned to laminated composites comprised of fiberglass fabric impregnated with fluorocarbon resins. These laminated composites have a dielectric constant of approximately 2.5 at 1 megahertz. However, fluorocabrons are not thermosetting resins and are extremely difficult to fabricate into multilayer printed circuit boards. At temperatures at which the fluorocarbon pre-preg sheets will bond the package together, the innerlayers can melt or lose their dimensional stability. If FR-.sub.4 pre-pregs are used to bond the fluorocarbon innerlayers together, the resulting multilayer composite is non-homogeneous and the dielectric constant is raised as a result of the higher dielectric constant of the pre-preg.
Other specialty dielectric materials have been developed which utilize fibers other than fiberglass in combination with thermosetting resins. Laminated composites of polyaramide fibers and epoxy resins have a dielectric constant of 3.8, which is considerably higher than the fluorocarbon composites. Quartz fibers have been used in composites, but quartz fibers have nearly the same dielectric constant as polyaramide fibers.